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<div class="section" id="placeholder-expression">
<h1><a class="toc-backref" href="./metafunctions-concepts.html#id1515">Placeholder Expression</a></h1>
<div class="section" id="id835">
<h3><a class="subsection-title" href="#description" name="description">Description</a></h3>
<p>A <a class="reference internal" href="./placeholder-expression.html">Placeholder Expression</a> is a type that is either a <a class="reference internal" href="./placeholders.html#placeholder">placeholder</a> or a class
template specialization with at least one argument that itself is a
<a class="reference internal" href="./placeholder-expression.html">Placeholder Expression</a>.</p>
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<div class="section" id="id836">
<h3><a class="subsection-title" href="#expression-requirements" name="expression-requirements">Expression requirements</a></h3>
<p>If <tt class="literal"><span class="pre">X</span></tt> is a class template, and <tt class="literal"><span class="pre">a1</span></tt>,... <tt class="literal"><span class="pre">an</span></tt> are arbitrary types, then
<tt class="literal"><span class="pre">X&lt;a1,...,an&gt;</span></tt> is a <a class="reference internal" href="./placeholder-expression.html">Placeholder Expression</a> if and only if all of the following
conditions hold:</p>
<ul class="simple">
<li>At least one of the template arguments <tt class="literal"><span class="pre">a1</span></tt>,... <tt class="literal"><span class="pre">an</span></tt> is a <a class="reference internal" href="./placeholders.html#placeholder">placeholder</a>
or a <a class="reference internal" href="./placeholder-expression.html">Placeholder Expression</a>.</li>
<li>All of <tt class="literal"><span class="pre">X</span></tt>'s template parameters, including the default ones, are types.</li>
<li>The number of <tt class="literal"><span class="pre">X</span></tt>'s template parameters, including the default ones, is
less or equal to the value of <tt class="literal"><span class="pre"><a href="./limit-metafunction-arity.html" class="identifier">BOOST_MPL_LIMIT_METAFUNCTION_ARITY</a></span></tt>
<a class="reference internal" href="./configuration.html">configuration macro</a>.</li>
</ul>
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<div class="section" id="id838">
<h3><a class="subsection-title" href="#models" name="models">Models</a></h3>
<ul class="simple">
<li><a class="reference internal" href="./placeholders.html">_1</a></li>
<li><tt class="literal"><span class="pre"><a href="./plus.html" class="identifier">plus</a>&lt;_,</span> <span class="pre"><a href="./int.html" class="identifier">int_</a>&lt;2&gt;</span> <span class="pre">&gt;</span></tt></li>
<li><tt class="literal"><span class="pre"><a href="./if.html" class="identifier">if_</a>&lt;</span> <span class="pre"><a href="./less.html" class="identifier">less</a>&lt;<a href="./placeholders.html" class="identifier">_1</a>,</span> <span class="pre"><a href="./int.html" class="identifier">int_</a>&lt;7&gt;</span> <span class="pre">&gt;,</span> <span class="pre"><a href="./plus.html" class="identifier">plus</a>&lt;<a href="./placeholders.html" class="identifier">_1</a>,<a href="./placeholders.html" class="identifier">_2</a>&gt;,</span> <span class="pre"><a href="./placeholders.html" class="identifier">_1</a></span> <span class="pre">&gt;</span></tt></li>
</ul>
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<div class="section" id="id839">
<h3><a class="subsection-title" href="#see-also" name="see-also">See also</a></h3>
<p><a class="reference internal" href="./lambda-expression.html">Lambda Expression</a>, <a class="reference internal" href="./placeholders.html">Placeholders</a>, <a class="reference internal" href="./metafunctions.html">Metafunctions</a>, <a class="reference internal" href="./apply.html">apply</a>, <a class="reference internal" href="./lambda.html">lambda</a></p>
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